Recently, a technique for forming images, such as characters or figures printed on a recording medium such as a sheet, by an erasable colorant has been proposed. An image is formed on a sheet by such an erasable colorant, and the image is then erased. With the repetition of this cycle, one sheet can be reutilized several times, thus being able to contribute to resource-saving.
To indicate to a user that this erasable colorant is erasable, for example, a specific color such as blue is sometimes used.
In case an image of an erasable colorant printed on a sheet is read out by an image reader, the image of the erasable colorant of a specific color is sometimes read with low contrast in accordance with light source for illuminating an original which is used in the image reader.
The image reader reads a reflecting image of an original surface illuminated by a light source by an image sensor such as CCD line sensor. If a white light source is used as the light source, an erasable image with a blue color is read with low contrast due to the influence of a blue component which is included in the white light source.